1. Field of the Invention
This invention relates to crystalline boehmitic aluminas the crystallites of which exhibit unusual dimensional differences in the space directions 020 and 120. This invention further relates to a method for preparing such aluminas and the follow-up products obtained therefrom by calcination.
2. Description of the Prior Art
The structural relations of the various aluminium oxides and aluminium hydroxides are very complex. Main distinctions are made between xcex1-Al2O3 (corundum), xcex1-AlO(OH) (diaspore), xcex1-Al(OH)3 (occasionally also termed xcex2-Al(OH)3, bayerite, or bauxite dihydrate), xcex3-Al2O3, xcex3-AlO(OH) (boehmite), and xcex3-Al(OH)3 (occasionally also termed xcex1-Al(OH)3, gibbsite, hydrargillite). In addition, there exist numerous modifications thereof, particularly modifications of different aluminium oxides obtained by thermal degradation of the aluminium hydroxides or aluminium oxide hydrates. For instance, it is generally believed that boehmitic alumina will undergo the following conversion under the influence of temperature:
Boehmitexe2x86x92xcex3(gamma)-Al2O3xe2x86x92xcex4(delta)-Al2O3xe2x86x92xcex8(theta)-Al2O3xe2x86x92xcex1(alpha)-Al2O3
In literature references there are no standardized designations for the various aluminium oxides, aluminium oxide hydrates (occasionally also termed aluminium oxide hydroxides), and aluminium hydroxides, particularly with respect to the preceding Greek characters. The term xe2x80x98boehmitic aluminasxe2x80x99 as used herein comprises boehmitic and pseudo-boehmitic aluminas.
Boehmitic aluminas are known. High-purity boehmitic aluminas can be prepared for example by controlled hydrolysis of aluminium alkoxides. The resultant aluminium hydroxide hydrogels crystallize for example in the form of the rhombic aluminium oxide hydrate crystallite (xcex3-AlO(OH), boehmitic alumina).
DE 38 23 895-C1 discloses a process for producing boehmitic aluminas with pore radii which can be adjusted in a controlled way from 3 to 100 nm. According to said process, the boehmitic aluminas are subjected to hydrothermal aging at a steam pressure from 1 to 30 bar (corresponding to a temperature from 100 to 235xc2x0 C.) for 0.5 to hours with agitation at a peripheral velocity from 1 to 6 sxe2x88x921. Such aluminas and the boehmitic aluminas produced by other processes have crys-tallite sizes (measured on the 020 reflex) which are always smaller by at least 2 nm compared to the crystallite sizes measured on the 120 reflex. In U.S. Pat. No. 3,898,322, too, a process for producing hydrothermally aged alumina suspension is described. According to said process, the aqueous aluminium hydroxide/aluminium oxide hydrate suspension obtained by hydrolysis of the aluminium alkoxides is subjected to hydrothermal aging at room temperature for 2 to 60 hours.
It is an object of the present invention to provide boehmitic aluminas having unusual morphologies. It is a further object of this invention to provide aluminium oxides with unusual high-temperature stabilities and, furthermore, with extraordinarily large surfaces and pore volumes after calcination.
The problem is solved by crystalline boehmitic alumina with a crystallite size measured in nm on the 020 reflex which is larger than the crystallite size which is smaller by 1.5 nm, preferably 0.5 nm, measured on the 120 reflex. It is particularly preferred that the crystallite size measured in nm on the 020 reflex be larger than the crystallite size measured in nm on the 120 reflex.
It is a further object of the present invention to provide methods for preparing the crystalline boehmitic aluminas of the instant invention.
The starting compounds employed for preparing the crystalline boehmitic aluminas of the present invention are conventional (i.e. crystalline, partially crystalline, or amorphous) aluminium-oxygen compounds, such as aluminium oxide hydrates, aluminium hydroxides, or mixtures thereof with aluminium oxides, preferably conventional pseudoboehmitic and/or boehmitic aluminas. When using commercial aluminium-oxygen compounds produced by other processes, or when the boehmitic alumina is not produced and is readily employed, for example as a hydrogel, it is preferred that the aluminium-oxygen compounds be subjected to grinding prior to aging according to this invention.
The starting compounds are aluminium oxide hydrates (or aluminium oxide hydroxides) which are preferably prepared by hydrolysis of aluminium alkoxides obtained from C1 to C24+ alcohols or mixtures thereof. The aluminium alkoxides can be prepared for example by the Ziegler process.
The aluminium alkoxides are hydrolyzed in an aqueous environment. Generally, the hydrolysis can be performed in a temperature range from 30 to 150xc2x0 C., preferably 60 to 100xc2x0 C. The resultant aluminium oxide hydrate suspension is then separated from the aqueous alcohol phase. The alumina-water phase may contain for example alumina hydrate with an Al2O3 content from 5 to 12 wt. %, preferably 10 to 11 wt. %.
The aluminium-oxygen compounds employed as starting materials may also originate from natural resources or can be produced by other processes, e.g. the amalgam process.
The crystalline boehmitic aluminas of the present invention can be prepared by long-time hydrothermal aging of oxygen compounds of the aluminium, particularly aluminium oxide hydrates, in the presence of water at temperatures from 60 to 240xc2x0 C., preferably 70 to 160xc2x0 C., most preferably 70 to 110xc2x0 C. for at least 10 hours, preferably at least 20 hours, most preferably at least 24 to 70 hours or 30 to 60 hours. It is desirable to keep the shear stress on the aluminium oxide hydrate suspension low during the production. The term xe2x80x98low shear stressxe2x80x99 used herein means the shear stress caused by an agitator, e.g. a propeller agitator, running at a peripheral velocity of 0.5 to 3 m/s. The particle size of the aluminium oxide hydrates in the suspension is preferably in the range from 1 to 12 microns, most preferably from 6 to 12 microns.
According to another embodiment of the present invention, the crystalline boehmitic aluminas of this invention can be prepared by hydrothermal aging in the presence of water and at least bidentate, preferably at least tridentate bases, which are preferably nitrogen bases, and at temperatures from 30 to 240xc2x0 C., preferably 70 to 160xc2x0 C., for 0.5 to 170 hours. Examples thereof are diethylene triamine, dipropylene triamine, triethylene tetramine (triene), tetraethylene pentamine (tetrene), and pentaethylene hexamine (pentrene).
According to yet another embodiment of the present invention, the crystalline boehmitic aluminas of this invention can be prepared by long-time hydrothermal aging in the presence of water and metallic or nonmetallic oxides, or oxide hydrates, except for aluminium oxide or aluminium oxide hydrates, and water at 40 to 240xc2x0 C., preferably 70 to 160xc2x0 C., for at least 8 hours, preferably 16 to 170 hours, most preferably 32 to 170 hours.
Preferably, said metallic or nonmetallic oxides or oxide hydrates are those of silicon, zirconium, titanium, lanthane, and/or boron. Examples thereof are SiO2, ZrO2, TiO2, and B2O3. Such oxides are added in quantities from 0.1 to 5 wt. %, preferably 0.2 to 2 wt. %, referring to A12O3.
It is preferred that the crystalline boehmitic aluminas of the present invention or the aluminium oxides prepared therefrom be free from any foreign atoms, particularly other metal atoms (including silicon and phosphorus), i.e. said materials should exclusively consist of aluminium, oxygen, and/or hydrogen in quantities of greater than 99 atom %, preferably greater than 99.9 atom %.
Preferably, the crystalline boehmitic aluminas of the present invention, independently of one another, have the following characteristics: pore volumes of greater than 0.8 cm3/g, preferably greater than 0.9 cm3/g, crystallite sizes (measured on the 020 reflex) of greater than 10 nm, preferably greater than 12 nm, and surfaces of greater than 150 m2, preferably 150 to 200 m2. In contrast thereto, it is preferred that the crystalline boehmitic aluminas of the present invention prepared according to the third embodiment, independently of one another, have the following characteristics: pore volumes of greater than 0.7 cm3/g, preferably greater than 0.9 cm3/g, crystallite sizes (measured on the 020 reflex) from about 6 to 10 nm, and surfaces of greater than 200 m2.
The aluminium oxides prepared from the crystalline aluminas of the present invention by thermal treatment at higher than 150xc2x0 C., preferably by calcination at temperatures from 800 to 1,500xc2x0 C. for at least 0.5 hour are a further object of the present invention. Said aluminium oxides are distinguished by their particularly large surfaces, large pore volumes, and excellent high-temperature stabilities. The term xe2x80x98thermal stabilityxe2x80x99 employed herein means stability to changes in the surface or crystalline phase brought about by external influences, such as water, chemicals, pressure, or mechanical stress and temperature.
Furthermore, the aluminium oxide hydrates and aluminium oxides according to the present invention are pure-phase and stable-phase products which are present as delta, theta, or alpha modifications, depending on the calcination time and temperature. More details are presented in the tables 1, 2, and 3 showing the powder diffraction pattern data of the different aluminium oxides of this invention.
The term xe2x80x98pure-phasexe2x80x99 employed herein means that more than 90 wt. %, preferably more than 98 wt. % of the crystalline aluminium oxide consists of a single phase (determined by X-ray powder diffraction). The theta-aluminium oxides of the present invention are pure-phase products on the condition that particularly the d-values (as xc3x85) do not present any peaks in the X-ray powder diffraction pattern which are characteristic of xcex1-Al2O3.
With respect to the X-ray powder diffraction patterns of conventional aluminium oxides, reference is made to the corresponding. JCPDS sheets (US National Bureau of Standards) for corundum (xcex1-Al2O3), delta- and theta-aluminium oxide.
The term xe2x80x98stable-phasexe2x80x99 employed herein means that the crystalline phase will not change even if the product is exposed for a long time to the same or lower temperature used in the production of said aluminium oxide by calcination.
Furthermore, the aluminium oxides of the present invention are temperature-stable and, contrary to conventional aluminium oxides, have surfaces of larger than 60 m2/, preferably larger than 70 m2/g, even after calcination at 1,200xc2x0 C. for 3 hours. The calcination is performed in heated air in a muffle furnace.
The aluminium oxides of the present invention have pore volumes of greater than 0.6 cm3/g, preferably from 0.7 to 1 cm3/g (determined by the mercury penetration method in accordance with DIN 66 133) within a pore radius range from 1.8 to 100 nm. The aluminium oxides of this invention keep said characteristic even after exposure to temperatures of 1,100xc2x0 C. for 24 hours. Conventional aluminium oxides, e.g. those obtained by calcination of bayerite, present distinctly smaller pore volumes (about 0.2 to 0.4 cm3/g).
The aluminium,oxides of this invention are most useful as catalysts or catalyst supports, particularly as support material for automobile exhaust gas catalysts. In this case the catalyst support is treated with noble metal catalysts, such as platinum or palladium.
When using the aluminium oxides of this invention, the catalyst or catalyst support can be applied in thin layers which remain stable even at high temperatures, e.g. of greater than 1,000xc2x0 C. This characteristic is most advantageous in exhaust gas catalyst applications. Furthermore, stabilization aids, such as lanthane oxideor SiO2, employed in technical applications can mostly be dispensed with. Stabilization aids made of metal oxides may have adverse effects on the catalytic behavior of the Al2O3 catalyst or catalyst support.
The crystallite sizes of the boehmitic aluminas according to this invention were determined on the 120 and 020 reflexes using the general Scherrer formula:
xe2x80x83Crystallite size=(Kxc3x97lambdaxc3x9757.3)/(betaxc3x97cos theta)
K (form factor): 0.992
Lambda (X-ray wave length): 0.154 nm
Beta (corrected line broadening of apparatus): reflex-dependent
Theta: reflex-dependent
The measurements were carried out using a Philips XRD X""pert apparatus. The measurement parameters for the samples obtained in Example 1 (Comparative Example) and Example 2 have been compiled in Tables 1 and 2, respectively.
The reflexes 120 and 020 (Miller indices) were determined on the boehmite and relate to the unconventional crystallographic Amam mounting of orthorhombic space group no. 63. The conventional mounting is Cmcn, wherein the a- and c-axes have been exchanged in comparison with the unconventional Amam mounting.
The surface areas of the aluminium oxides of this invention were determined by the N2 sorption method (BET method in accordance with DIN 66131). The pore volumes and pore volume distributions were determined by the mercury intrusion (penetration) method in accordance with DIN 66133 using a mercury porosimeter. The pore volumes were reported as cumulative volumes in cm3/g in accordance with DIN 66133.